Improperly formed request.

### Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my issue
- [x] I have run `q doctor` in the affected terminal session
- [x] I have run `q restart` and replicated the issue again

### Operating system

macOS 15.5.0 (24F74)

### Expected behaviour

Q CLI should answer

### Actual behaviour

```
Amazon Q is having trouble responding right now:
0: unhandled error (ValidationException)
1: service error
2: unhandled error (ValidationException)
3: Error { code: "ValidationException", message: "Improperly formed request.", aws_request_id: "725abae4-7a6d-4922-a0e8-c7c6499c1b89" }

Location:
crates/chat-cli/src/cli/chat/mod.rs:782

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.
```

### Steps to reproduce

q chat
> Hello
>

```
Amazon Q is having trouble responding right now:
0: unhandled error (ValidationException)
1: service error
2: unhandled error (ValidationException)
3: Error { code: "ValidationException", message: "Improperly formed request.", aws_request_id: "725abae4-7a6d-4922-a0e8-c7c6499c1b89" }

Location:
crates/chat-cli/src/cli/chat/mod.rs:782

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.
```

### Environment

```yaml

[q-details]
version = "1.12.2"
hash = "b9100eb751546269f095585705a0e7fc3ea55419"
date = "2025-06-30T22:40:01.105251Z (3d ago)"
variant = "full"

[system-info]
os = "macOS 15.5.0 (24F74)"
chip = "Apple M1"
total-cores = 8
memory = "16.00 GB"

[environment]
cwd = "/Users/USER"
cli-path = "/Users/USER"
os = "Mac"
shell-path = "/bin/bash"
shell-version = "3.2.57"
terminal = "macOS"
install-method = "unknown"

[env-vars]
PATH = "/Users/USER/miniforge3/condabin:/usr/local/bin/mwinit:/opt/homebrew/bin:/usr/local/sbin:/usr/local/bin:/usr/local/bin:/System/Cryptexes/App/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin:/Library/Frameworks/Mono.framework/Versions/Current/Commands:/Users/USER/.local/bin"
QTERM_SESSION_ID = "c189df0da53741f2af35037c54fa9919"
Q_SET_PARENT_CHECK = "1"
Q_TERM = "1.12.2"
SHELL = "/opt/homebrew/bin/bash"
TERM = "xterm-256color"
__CFBundleIdentifier = "com.apple.Terminal"
```